1. Field of the Invention
The present invention relates to a method for transmitting data signals on analog subscriber lines between subscriber terminal equipment and a telephone exchange, preferably during the speech signal transmission of an existing telephone connection.
2. Description of the Prior Art
A variety of different methods are applied for transmitting data on analog subscriber lines that primarily serve the speech signal transmission between a piece of telephone terminal equipment and the telephone exchange. Among them is a DC signalization wherein the data signal transmission occurs by means of sequences of loop interruptions and a loop signalization that is balanced, or imbalanced, to ground is provided. An AC signalization is also suitable wherein AC signals of 50 Hz are coupled onto the subscriber line loops in ground-imbalanced fashion. Another type of data transmission is what is known as DTMF signalization (multi-frequency code signalization), wherein the signaling information is formed by means of an m out of n coding of n tone frequency signals. Finally, there are what are known as modem transmissions, for which the binary data signals are converted into technically advantageous line signals which correspond to the electrical conditions of analog connections; i.e., frequency modulation, phase difference modulation and quadrature amplitude modulation being essentially suitable for this purpose. The DC signalization is disadvantageous since it disturbs the speech transmission. The AC signalization is extremely slow and thus suitable only for certain types of data transmissions. The two latter methods for data exchange use the speech frequency range and are thus unsuitable for a simultaneous transmission of data and speech. However, there is a need for this, particularly in connection with functions offered by modem convenience subscriber terminal equipments.
It is thus an object of the present invention to propose a method for the transmission of data signals on analog subscriber lines wherein such transmission can also occur during the speech signal transmission and can be implemented with relatively low outlay.